1. Field of the Invention
The present invention relates to a process for producing propylene-ethylene block copolymers. More specifically, the present invention relates to a process for producing propylene-ethylene copolymers having improved properties by using highly active composite catalyst systems.
2. Description of the Prior Art
Since the invention of stereoregular catalysts by Ziegler and Natta, crystalline polyolefins have been produced on an industrial scale. Crystalline polypropylene has attracted special attention as a general-purpose resin having excellent stiffness and heat resistance. However, because crystalline polypropylene is brittle at low temperatures it is not suitable in uses which require low temperature impact resistance. In order to overcome this disadvantage, extensive studies have been directed to improved processes for preparing the polyolefin. Among them, processes for block copolymerizing propylene with other olefins, especially ethylene, are known. Japanese Patent Publication Nos. 13049/68 and 26113/72, Japanese Patent Application (OPI) Nos. 120986/74, 135987/76 and 3684/77 propose block copolymerization processes comprising, in the first stage, polymerizing propylene alone in the liquefied propylene in the absence of an inert solvent and, successively in the second stage, randomly copolymerizing ethylene and propylene in liquefied monomer or in monomer gas or polymerizing ethylene alone in monomer gas.
When conventional catalyst systems consisting essentially of titanium trichloride solid catalyst (such as titanium trichloride obtained by reducing titanium tetrachloride with metallic aluminum or hydrogen or co-crystallization products of titanium trichloride and aluminum trichloride, and an organoaluminum compound) are used in the propylene-ethylene block copolymerization, a large amount of catalyst residue remains in the polymer particle product because the amount of polymer produced per gram of titanium trichloride solid catalyst is small. As a result when the polymer product is molded into an article it yellows thus deteriorating its commercial value.
In addition, as polymer particles are usually melt mixed and pelletized and further molded by melting in various molding machines. Very often these polymers are remarkably degraded during such melting step and suffer a loss in mechanical properties. Further, degradation by ultraviolet light is so marked that the use of moldings for an extended time is difficult. Accordingly, in such processes, it is necessary to post treat the polymer such as by extraction of the catalyst residues, etc. as is necessary in the polymerization in an inert solvent, thus complicating the production.
Furthermore, because the solubility of low molecular weight and low crystalline polymer by-products is higher in the inert solvents commonly employed in some polymerizations than in the liquefied monomer, when the propylene-ethylene block copolymerization is conducted in the absence of the inert solvent, the proportion of the low molecular weight and low crystalline polymer contained in the polymer particles is remarkably higher as compared with the case of the polymerization in the inert solvent. As a result, if a conventional catalyst is employed, the characteristics of the propylene polymers, i.e., stiffness and heat resistance, suffer and it is difficult to use moldings in fields where such physical properties are required. The low molecular weight and low crystalline polymer by-product forms on the polymer particle surfaces in great amounts and the cohesion between the particles increases and problems such as agglomeration of the polymer, reduction in heat transfer efficiency due to the deposit of the polymer on the inner walls of a polymerization vessel and even frequent occurance of blocking of pipes, powder hoppers and silos during transportation of the polymer slurry or polymer powder occur.
In conducting the polymerization in the gas phase, as proposed in Japanese Patent Publication Nos. 597/66 and 13962/72, Japanese Patent Application (OPI) No. 145589/76 etc. a fluidized-bed reactor fluidized with the monomer gas or an agitating fluidized-bed reactor is useful to remove the polymerization heat and prevent the polymer particles from agglomerating or melting. In the case of the polymer particles having great cohesion, extremely large amounts of fluidizing gas flow or stirring force are needed to fluidize the polymer particles. If the cohesion is still too great, it is not possible to keep uniform fluidized conditions, leading to the difficulty in uniform heat removal and to agglomeration or melting of the polymer. Thus production is jeopardized.
Further, if a large amount of the low molecular weight and low crystalline polymer is contained in the polymer particles, the stiffness and heat resistance which are characteristic of polypropylene deteriorate remarkably. Therefore, it is necessary to use catalyst systems which reduce the production of such low molecular weight and low crystalline polymer if the production of some low molecular weight and low crystalline by-product cannot be avoided and prevent the increase in cohesion between the particles when propylene and ethylene are block copolymerized in the substantial absence of an inert solvent in the liquefied monomer or in the gas phase.
Moreover, as mentioned above, in the case of the polymerization in the gas phase, although it is effective to circulate the monomer gas, if the particle size distribution of produced polymer particles is broad and fine particles are present in fairly large amounts, flying of fine particles occurs and much effort and uneconomical equipment are required to separate or remove these fine particles, otherwise they could block a heat exchange or circulating compressor. Therefore, the particle size distribution of the polymer particle product is desirably as narrow as possible. Thus, in conducting the process of the present invention for producing propylene-ethylene block copolymers, it is essential to use catalyst systems which reduce the production of such low molecular weight and low crystalline polymer and, if more or less produced, provide polymer particles having less cohesion and narrower particle size distribution.
However, if the polymerization similar to that of the present invention is conducted in the absence of an inert solvent using the catalyst systems disclosed in the prior art mentioned hereinbefore, the various problems mentioned above occur because the polymer particles contain the low molecular weight and low crystalline polymer. Similarly, in Japanese Patent Publication No. 20501/64, Japanese Patent Application (OPI) No. 1046/71 etc., there are proposed processes for producing titanium trichloride compositions which contain only small amounts of fine particles and give polymers in a narrow particle size distribution by reducing titanium tetrachloride with an organoaluminum compound and further treating the catalyst with heat. Even when such catalysts are used to produce propylene-ethylene block copolymers as in the present invention, the produced polymers, although having a narrow particle size distribution, contain large amounts of the low molecular weight and low crystalline polymer by-product which remarkably increases cohesion between the polymer particles. In addition, the catalyst's polymerization activity and stereo-regularity are unsatisfactory. Thus, it has actually been very difficult to produce propylene-ethylene block copolymers under stable conditions.
As a result of extensive research directed to overcoming the aforementioned problems, a process for stably and economically producing propylene-ethylene block copolymers having improved properties without the above problems has been found.